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Figure 2. Pulsed cathodic arc at the University of Sydney. Figure 4. Al/C pulse ratio vs the Al/C film composition ratio from SNMS. The linear relationship establishes a quick and reliable method for accurately controlling and quantifying the composition of thin film MAX phases.
Figure 4. Al/C pulse ratio vs the Al/C film composition ratio from SNMS. The linear relationship establishes a quick and reliable method for accurately controlling and quantifying the composition of thin film MAX phases.
Synthesis and characterisation of thin film MAX phase alloys
Mathew Guenette, Mark Tucker, Yongbai Yin, Marcela Bilek, David McKenzie
Applied and Plasma Physics, University of Sydney 2006, Australia.
MAX phases are a family of ternary alloys that exhibit a unique combination of metallic and ceramic properties. That is, machinablility, resistance to thermal shock, high thermal and electrical conductivity combined with stiffness, oxidation resistance and high temperature (T>1400°C) stability.
The nanolaminate crystal structure of the MAX phases consists of single hexagonal layers of the A element between metal-carbide or metal-nitride (Mn+1Xn) layers.
H. Högberg et al. Surface & Coatings Technology 193: 6–10, 2005
Figure 1. General Mn+1AXn phase composition (n=1,2,3) and unit cell
XRD scans of near stoichiometric Ti2AlC reveal a highly oriented, relatively phase pure thin film. The θ/2θ scan in Fig.5 shows strong reflections only from Ti2AlC(00ℓ) planes demonstrating preferred c-axis oriented film growth. Oriented TiC(hhh) reflections are also observed. The glancing angle scan reveals only Ti2AlC peaks demonstrating a highly phase pure film.
Pulsed cathodic arc MAX phase synthesis
and SNMS composition analysis
Deposition of Ti2AlC thin film on Al2O3(0001) at a temperature of 900°C was carried out using a high current, centre triggered, pulsed cathodic arc fitted with three cathodes (Ti, Al and C) (Fig. 2). The film composition can be accurately controlled by varying the number of pulses of each element. A linear relationship between the number of pulses from the cathodes and the composition of the film is observed over the order of 10 000 to 100 000 pulses (Fig. 4).
Due to cathode erosion with deposition, long term drift in the composition of deposited films occurs. Film composition is affected by changes in substrate temperature and bias due to different mass and volatility between elements. Correct stoichiometry is vital to successfully synthesise MAX phases and a method for rapid tuning of film composition is required.
Figure 3. SNMS chamber
Secondary Neutral Mass Spectrometry (SNMS) is a fast and convenient plasma technique which can be used to analyse the composition of a thin film. The film is sputtered with an argon plasma and the composition of the emerging neutral particles is measured with a mass spectrometer. The elemental ratio within the sample is directly proportional to the ratio of the SNMS signal intensities. Quantitative analysis of films is possible by comparison to standards of known composition.
Figure 5. a) θ/2θ XRD scan b) glancing angle XRD scan (incident angle = 0.6°)